Last April, the Justice Department sued Apple and five major book publishers, accusing them of a conspiracy to drive up the prices of digital books.

In the lawsuits filed by federal and state officials, Apple co-founder and former CEO Steve Jobs was named as a key player who helped orchestrate a complex price-fixing plan that cost consumers tens of millions of dollars over two years. Federal investigators said the prices of many new releases and bestsellers had been boosted by $2 to $5 each.

Shortly after the suits were filed, Apple vehemently denied wrongdoing, saying the Justice Department's "accusation of collusion against Apple is simply not true."

The five book publishers — Simon & Schuster, Hachette Book Group, HarperCollins Publishers, Macmillan and Penguin Group — also denied the allegations but reached settlements with the Justice Department.

Cote said in a teleconference Wednesday that Jobs' 2011 death was a key reason she was ordering Cook to testify.

The government is not requesting damages in the trial, slated to begin in June, but is seeking a finding that Apple violated antitrust law. It also wants an order blocking the Cupertino firm from engaging in similar conduct in the future, Reuters said.